The invention is directed to an apparatus and a method for measuring the radioactivity of an eluate by a flow-through detector arrangement.
It is known in radiochromatography to measure the radioactivity of the liquid eluate (eluation agent) carrying the radioactive fraction by means of a flow-through detector, through which eluate flows.
The measurement of the radioactive radiation is herein subject to the statistical fluctuation of the emission of the radiation particles. The statistical error caused hereby corresponds to the square root of the quantity of the measured events (Sigma=.sqroot.n, wherein Sigma designates the statistical error and n the total quantity of the measured events). 95.5% of all measured values lie within the limits of .+-.2 Sigma, which values can be acquired with an average value of n.
When measuring radioactivity in a flow-through detector, n events per second are registered. If one multiplies this number by the dwelling time of the radioactively marked fraction in the flow-through measuring cell, then one gets the total number of the events acquired during the flow-through process. The dwelling time in the measuring cell of the radioactively marked fractions flowing through the measuring cell is computed from the quotient of measuring cell volume divided by the flow velocity.
In order to reduce the statistical measuring error, it could be considered to lengthen the measuring cell so that the transit period is correspondingly increased and the quantity of the measured events is increased. For instance, if the measuring cell were twice as large, the transit period would be doubled and with this also the quantity of the measured events would become twice as large, so that the specific error Sigma (.sqroot.n) would be correspondingly reduced. Such an increase in the size of the measuring cell leads however to a corresponding increased fabrication cost and also reduces the resolution ability of the measuring cell, since radioactive fractions following rapidly upon each other can no longer be measured selectively, since the previous radioactive fraction can still be in the measuring cell while the subsequent radioactive fraction is already flowing into the same measuring cell. The measuring result is then composed of the radiation results of both radioactive fractions together, so that it is no longer possible to discriminate clearly or no longer possible to discriminate at all between the radioactive fractions and their associated radioactive peaks.
On the other hand a detection sensitivity in measuring cells with the usual volume size is not very high, since due to the relatively high error fluctuation width, a very weak radioactivity rate cannot be reliably differentiated from the zero effect rate and its statistical fluctuation. This entails that the detection sensitivity limit, above which the presence of a radioactive fraction can even begin to be reliably registered, is relatively high.